In driving a rotary electric machine by an inverter, a control mode is switched over among a sinusoidal wave current control mode, an overmodulation current control mode and a rectangular wave voltage phase control mode. The rectangular wave voltage phase control mode, which uses one-pulse switching, is required to attain both high output power and size reduction of the rotary electric machine. The rotary electric machine is controlled optimally by switching over the control mode between the sinusoidal wave current control mode, which provides a superior characteristic in a low speed region, and the overmodulation current control mode, which is used in an intermediate rotation speed region.
In the sinusoidal wave current control mode and the overmodulation current control mode, current feedback control is performed to output a pulse-width modulation (PWM) pattern to the rotary electric machine by comparing a command voltage and a carrier wave voltage. In the rectangular wave voltage phase control mode, torque feedback control is performed to output a one-pulse switching waveform to the rotary electric machine in accordance with an electric angle by fixing an amplitude of the voltage to a maximum value and controlling phase.
In the sinusoidal wave current control mode and the overmodulation current control mode, torque control is performed in accordance with a current phase by generating a pseudo-sinusoidal waveform by PWM technology and using a vector control method. Here, in a d-q plane defined by a d-axis and a q-axis used in vector control, a relational equation is defined between a torque T and a current phase β between the d-axis current and the q-axis current, and a desired or target torque is produced by varying the current phase β in accordance with the defined relational equation.
By assuming that a relational equation derived by differentiating the torque T by the current phase β in this relational equation between the current phase β and the torque T, a relational equation of the current phase β for producing maximum torque can be defined. If the relational equation of the current phase β for producing the maximum torque is indicated on the d-q plane, a maximum efficiency characteristic line is determined by connecting current pairs, each of which is a pair of the d-axis current and the q-axis current providing maximum efficiency of the rotary electric machine. In the sinusoidal wave current control mode and the overmodulation current control mode, the rotary electric machine can be driven at the maximum efficiency by performing a current command on the maximum efficiency characteristic line, that is, by driving the rotary electric machine with a command current determined by the maximum efficiency characteristic line.
Control mode switching among the three control modes, that is, switching from the rectangular wave voltage phase control mode to the overmodulation current control mode and switching from the overmodulation current control mode to the rectangular wave voltage phase control mode, is performed in accordance with a rate of modulation or an amplitude of a command voltage corresponding to the modulation rate. The modulation rate is defined as a rate of an amplitude of a signal relative to an output voltage of an inverter.
For example, patent document 1 discloses that, in a control system for a permanent magnet-type synchronous electric motor, interphase basic wave voltages, which are required to control the torque under a condition that an output voltage Ed of the converter (inverter) Ed is constant, are defined as follows:    Ed×61/2/n=0.78×Ed, when the interphase basic wave voltage is a signal amplitude of a rectangular wave voltage; and    Ed×31/2/2×21/2=0.61=0.61×Ed, when the interphase basic wave voltage is a signal amplitude of the PWM method performed by comparing a sinusoidal wave and a triangular wave.
Further, patent document 2 discloses that a drive control system for an AC electric motor switches over a control mode as follows:    from a PWM current control mode to an overmodulation control mode, when a voltage amplitude exceeds 1.00 time of a peak value of a reference triangular wave;    from the PWM current control mode to a rectangular wave voltage phase control mode, when the voltage amplitude exceeds 1.27 times of the peak value of the reference triangular wave; and    from the rectangular wave voltage phase control mode to the overmodulation control mode, when an absolute value of a phase of an actual current becomes less than an absolute value of a phase of a command current.    Patent document 1: JP 11-299297A    Patent document 2: JP 2008-11682A (WO 2008/001524A1)
According to the PWM technology, the rotary electric machine can be driven to produce its torque at the maximum efficiency by controlling the command current on the maximum efficiency characteristic line. As disclosed in patent document 1, the modulation rate is 0.61 in the case of performing the PWM method by comparing the sinusoidal wave and the triangular wave. The modulation rate is 0.78 in the case of using the rectangular wave as the signal amplitude. For this reason, it is more appropriate to perform the rectangular wave voltage phase control, which can increase the modulation rate.
This means that, although the command current determined in accordance with the maximum efficiency characteristic line can drive the rotary electric machine at the maximum efficiency, it is not necessarily most efficient when an entire rotary electric machine system including the inverter is considered.